1. Field of the Invention
The present invention relates to an hydraulic drive system for a vehicle of the type having three independent wheel drive shaft assemblies in an essentially triangular configuration, namely a transversely arranged pair of wheel drive shaft assemblies and a single wheel drive shaft assembly longitudinally spaced from the transversely arranged pair. The drive system comprises an hydraulic pump and associated oil supply sump and a drive unit directly coupled to each of the drive shaft assemblies so that the drive units can be distinguished from each other, the drive unit connected to the single wheel drive shaft assembly is hereinafter called the xe2x80x9csinglexe2x80x9d drive unit and each drive unit connected to one of the transversely arranged pair of wheel drive assemblies is called a xe2x80x9ctransversexe2x80x9d drive unit. Each drive unit comprises at least two separate drives directly coupled to each drive shaft assembly, each drive unit having a separate hydraulic input and output and hydraulic feed lines for parallel operation and includes for series operation secondary hydraulic feed lines one between one of the drive units of the single drive unit and one of the drive units of one of the transverse drive units and the other between the other drive of the single drive unit and one of the drive units of the other transverse drive unit. The drive system further comprises a hydraulic drive controller, which controller comprises a diverter valve connected to each secondary hydraulic feed line and across each of the said drive units of the transverse drive units to provide parallel and series operation. It also relates to a steering system for such an hydraulic drive system.
2. Background Information
In European Patent Specification No. EP0816153 there is claimed a hydraulics drive system for a vehicle, having three independent wheel drive shaft assemblies in an essentially triangular configuration, namely a transversely arranged pair of wheel drive shaft assemblies and a single wheel drive shaft assembly longitudinally spaced from the transversely arranged pair. The drive system comprises a hydraulic pump and a drive unit directly coupled to each of the drive shaft assemblies again referred to as a single drive unit and a pair of transverse drive units. Each drive unit comprises at least two separate drives directly coupled to each drive shaft assembly. Each drive has a separate hydraulic input and output. Valving is provided to feed hydraulic fluid to each drive unit whereby the drive units operate in series or in parallel.
This is a particularly satisfactory method of operating the hydraulic drive system in that in almost all situations it eliminates the difficulties of skidding, wheel spin and the like because when using a series arrangement all the time a simple and effective drive system is provided. This is particularly effective for three-wheeled vehicles and for three wheeled vehicles of a relatively short wheel base. A problem arises when it becomes necessary to turn the vehicle when the drives are operating in series. This is usually achieved by the simple expedient of converting the drive to parallel operation. This causes problems of wheel spin, loss of traction and cavitation and these problems are discussed in more detail below.
Previous attempts have been made to overcome the problems of the operation of three-wheeled vehicles such as, for example, by the use of an hydraulic drive system as described in French Patent Specification No. 2719001 (E.C.B. SARL). This patent specification acknowledges the need for safety reasons to ensure that the wheels are all synchronised and that if the distributional load is, for example, equally distributed over the three wheels, the hydraulic transmission operates relatively well but that on slopes and, indeed, in difficult terrain it can become dangerous to operate the vehicle. Further, this patent specification discloses the problems with the prior art in the use of flow dividers, electronic distributors and the like. This patent specification proposes an arrangement somewhat similar to that described in European Patent Specification No. 0816153, however, unfortunately a very elaborate desynchronisation unit is provided comprising an electro-distributor which allows the flow of oil between the drives. If, with the electro-distributor shut, any of the wheels are locked or stopped for any reason with another wheel turning, excessive oil pressure can build up.
In this latter patent specification there is provided a hydraulic transmission which provides permanent synchronisation and then total de-synchronisation on negotiating a corner by transferring oil between the transverse drive assemblies. Unfortunately, in certain conditions particularly in poor terrain there may not be sufficient drive imparted to the outer wheels of the transverse drive assembly to negotiate the bend. Thus, certain problems have arisen in connection with the operation of a vehicle with an hydraulic drive system in accordance with the invention described and claimed in this European Patent Specification No. EP 0816153 in the sense that it must always be appreciated that situations will arise, particularly when working in difficult terrain, when one of the wheels for example is stopped or turning more slowly than the others. While this does not generally happen due to the arrangement there are always situations when it may. This has unfortunately caused certain problems in operation.
The invention is directed towards providing efficient steering without undue tyre wear and therefore a hydraulic drive system which operates satisfactorily as the vehicle is steered out of in-line travel such as around corners.
According to the invention there is provided a steering system for an hydraulic drive system for a vehicle of the type having three independent wheel drive shaft assemblies in an essentially triangular configuration, namely a transversely arranged pair of wheel drive shaft assemblies and a single wheel drive shaft assembly longitudinally spaced from the in-line pair. The drive system comprises an hydraulic pump and associated oil supply sump and a drive unit directly coupled to each of the drive shaft assemblies. So that the drive units can be distinguished from each other, the drive unit connected to the single wheel drive shaft assembly is hereinafter called the xe2x80x9csinglexe2x80x9d drive unit and each drive unit connected to one of the transversely arranged pair of wheel drive assemblies is called a xe2x80x9ctransversexe2x80x9d drive unit. Each drive unit comprises at least two separate drives directly coupled to each drive shaft assembly. Each drive has a separate hydraulic input and output and hydraulic feed lines for parallel operation and includes for series operation secondary hydraulic feed lines one between one of the drive units of the single drive unit and one of the drive units of one of the transverse drive units and the other between the other drive of the single drive unit and one of the drive units of the other transverse drive unit. The drive system further comprises a hydraulic drive controller, which controller comprises a diverter valve connected to each secondary hydraulic feed line and across each of the said drive units of the transverse drive units to provide parallel and series operation. The essential feature of the hydraulic drive system is that it comprises a steering control system including means for detecting movement of the vehicle from in-line travel so that on the vehicle moving out of in-line travel and steering around a particular transverse drive assembly, the single drive unit operates in parallel with that transverse drive unit and in series with the other transverse drive unit.
In normal operation when the drives are operating in series this overcomes the problem of turning without switching the whole vehicle to parallel operation. Thus good traction will be maintained even when turning in slippery conditions, in that the vehicle now has the benefits of parallel operation with the drive going round the turn as it were and series operation and thus increased traction with the outer of the two drives on the outer wheels. Similarly if operating in parallel the invention gives increased traction for turning when it is required by switching to series operation on the outer wheels.
Ideally the hydraulic drive system incorporates a steering mechanism connected to the single drive shaft assembly for steering by pivoting the single drive shaft assembly. This is a particularly useful way of providing the steering system as it ensures optimum control.
Generally speaking in such a system the steering mechanism includes a substantially vertically arranged steering shaft mounting the single drive shaft assembly.
Ideally the means for detecting movement of the vehicle out of in-line travel includes pivot sensing means for the steering shaft. These are just particularly suitable ways of providing the present invention.
In one embodiment of the Invention the pivot sensing means comprises a pair of electrical switches each connected to a relay for operation of one of the diverter valves and a switch actuator for each switch operable on pivoting of the single wheel drive shaft assembly into a turning mode. Such a type as which is relatively easy to provide is robust and is relatively sensitive in use.
Ideally there is a time delay on the operation of the diverter valves on detecting movement of the vehicle out of in-line travel. This ensures that the diverter valves open gradually on detecting movement of the single drive shaft assembly out of in-line travel. By providing a time delay there is time to transfer from series to parallel operation on the turning and this ensures that the ride is smooth and that the change over does not cause wheel spin or wear.
Ideally to provide the necessary delay, the diverter valves deliver the hydraulic fluid through flow restrictors, which flow restrictors are preferably orifices mounted in a fluid line fed by the diverter valve. This has been found to be a particularly easy way of providing the necessary delays which can be anything between 0.5 and 3 seconds but is ideally of the order of 1 second between detecting movements of the vehicle out of in-line travel and the full operation of the diverter valve. We found in practice that the one second delay is sufficient to give a gentle transfer from series to parallel operation.
Positive displacement piston valves have been found as being particularly suitable for diverter valves and it has also been found preferable to have a pressure release valve in each secondary hydraulic feed line so as there can be no rotation particularly when turning.